arm: attribute virtual IDs to the cell cpus

arm: attribute virtual IDs to the cell cpus

To handle SMP guests, the cells need to be assigned virtual CPU IDs
through the VMPIDR register. For the moment, those IDs are simply
generated incrementally on each CPU.

This change will allow to use the same guest code in different cells.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
[Jan: add "arm_" prefix, uninline arm_cpu_virt2phys]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>